This is my practice exam question and I have no answer for it nor do I know how to go about this....any help is greatly appreciated.

An open-topped cylindrical water reservoir has a volume of .

The material used for the bottom of the reservoir costs $5/ and the material used for the vertical portion costs $8/ .

Determine the radius and the height of the cylinder so that the cost is a minimum.

They also provide me with these equations:

Volume =

surface are =